A. Field of the Invention
The present invention is directed to lightweight synthetic stone products such as cultured onyx and solid surface used for sinks, bathtubs, counter tops, shower pans, wall panels and other related products and parts.
B. Description of the Prior Art
Cultured onyx is a well known synthetic stone product. Cultured onyx is generally made with a high purity, aluminum trihydrate (ATH), having the chemical formula Al.sub.2 O.sub.3 .multidot.3H.sub.2 O. ATH has a specific gravity of 2.4. Cultured onyx products are made with a low color, clear, unsaturated ortho polyester resin and a peroxide-containing catalyst such as a commercially available methyl ethyl ketone peroxide (MEKP) product.
In the United States, polyester resins are commercially available from Interplastic corporation's Silmar Resins, Reichhold Chemicals, Ashland Chemicals, Cook Composite Products, Inc., and McWhorter, Inc. MEKP is commercially available from the Norac Company, Inc.; Witco Corporation; and Akzo.
Conventional cultured onyx is made by mixing the ATH with the unsaturated ortho-polyester resin and the catalyst, placing the mix into a mold and permitting the polymerization reaction to proceed until such time as the casting is hard enough to remove from the mold.
Cultured onyx differs from other synthetic stone products, such as cultured marble, in the high degree of translucency exhibited by cultured onyx. The refractive index of the ATH component matches well with that of the resin, with the opacity due to the small amounts of pigment added to achieve the stone-like veining patterns typical for onyx. The high purity and clarity of the ATH insures that the background color is as neutral as possible, thereby allowing manufacturers to modify their products to please as many customers as possible.
Other synthetic stone products, such as cultured marble, have been made using lightweight, hollow fillers. However, cultured onyx, unlike other synthetic stone products such as cultured marble, have not heretofore been made with lightweight, hollow filler materials due to the low color, high translucency requirements of cultured onyx. Known hollow filler particles, whether white, i.e., low color, or of other colors, have multiple interfaces for reflection and refraction, and these multiple interfaces function to increase opacity. Additionally, the air contained within the particles increases the opacity by ensuring that the inside surfaces do not "wet out", that is, cover the surface with a liquid. Because these inside surfaces do not wet out, the differential refractive index between the inside and outside surface is relatively high. The differential refractive index between two surfaces defines opacity, e.g., high differential refractive indices indicates a high degree of opacity.
Cultured marble is also a well known synthetic stone product. Cultured marble is made with calcium carbonate (CaCO.sub.3). In the United States, many manufacturers of cultured marble also manufacture cultured onyx. It is believed that approximately one-third of these manufacturers use lightweight fillers as a substitute for part of the CaCO.sub.3 component in their cultured marble production. However, it is believed that no successful lightweight filler material has been found to function as a replacement for any part of the ATH component of cultured onyx.
As is also well known, manufacturers of cultured marble and cultured onyx use molds to shape their products. Typically, in a 10-30 mil thick layer of gel coat, unfilled iso-Neo Pentyl Glycol polyester resin catalyzed with MEKP, is applied to a mold, usually by spraying before the resin is placed into the mold. In recent years, with popularity of products such as DuPont's Corian.RTM. for counter tops and sinks, and with the development of advanced, acrylic-modified polyester resins, many cultured marble manufacturers have begun to produce parts without the protective gel coat. Such castings are extremely stain resistant, and are referred to as "solid surface" materials. Solid surface materials are also produced in a wide variety of colors and visual effects.
Conventional cultured onyx has a weight of approximately 6.5 lbs. for a 1' square, 3/4" thick sample. In contrast, a sample of cultured marble, 1' square.times.3/4" thick, typically weighs approximately 8 lbs. The primary difference between the weight of conventional cultured onyx and conventional cultured marble is due to the low specific gravity of the ATH compared to the specific gravity of calcium carbonate, and the higher percent of resin typically used in a cultured onyx product. Even though conventional cultured onyx is lighter than conventional marble, a need exists for cultured onyx having a lighter weight than the conventional cultured onyx.
Previous attempts to produce lightweight cultured onyx by substituting other materials for all or part of the ATH component have failed for various physical or chemical reasons. For example, it is known that portions of the ATH component have been substituted with salts, such as sodium silicate or sodium chloride, corn starch or with reground polycarbonate. However, when a salt was substituted for ATH, the casting blistered in use, due to the dissolution of the salt particles. Also, the castings swelled, cracked and then turned opaque when corn starch was substituted for ATH. In the case of use of reground polycarbonate used as a substitution for the ATH component, a high lead content was detected as leaching into the water.